fix(docs): restore site footer after mystmd conversion (#801)

The mystmd migration ported the footer styling in assets/css/site.css
(the `.footer` / `.outer-grid` rules, including the site-title heading)
but never added the footer content part. In the book-theme the
site-wide footer is a content part wired via `site.parts.footer`, which
was missing from config/scientific-python.yml — so no `<footer>` was
rendered and the site title that lives inside it disappeared.

Add docs/footer.md (site title, maintainer blurb, and links) and wire it
up with `site.parts.footer`, matching the upstream scientific-python-myst
-theme layout the CSS already expects.

Assisted-by: ClaudeCode:claude-opus-4.8

Author: henryiii

docs/config/scientific-python.yml

@@ -2,6 +2,8 @@ version: 1
 
 site:
   template: book-theme
+  parts:
+    footer: footer.md
   options:
     hide_toc: false
     hide_footer_links: true

docs/footer.md

@@ -0,0 +1,38 @@
+<!-- rumdl-disable MD041 -->
+
+% This defines the site-wide footer; it is not parsed as a regular "page".
+% It is wired up in `config/scientific-python.yml`:
+% site:
+% parts:
+% footer: footer.md
+% The column layout is controlled by `.footer .outer-grid` in
+% `assets/css/site.css` (grid-template-columns: 3fr 3fr 4fr).
+
+:::::{grid} 1 1 3 3
+:class: outer-grid col-screen
+
+<!-- Project description -->
+
+::::{div}
+
+# Scientific Python Development Guide
+
+This guide is maintained by the scientific Python community for the benefit of
+fellow scientists and research software engineers.
+::::
+
+<!-- Spacer between description and links -->
+
+::::{div}
+::::
+
+<!-- Links -->
+
+::::{div}
+
+- [Scientific Python](https://scientific-python.org)
+- [Learn](https://learn.scientific-python.org)
+- [Source on GitHub](https://github.com/scientific-python/cookie)
+  ::::
+
+:::::